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IMPORTANT DESIGN FACTORS
Climate and regional applicability
Since WWG systems rely on green plants and microbes, they perform more rapidly in warm, sunny conditions, the approach is ideal for climates ranging from tropical to Mediterranean-type climates. In these conditions with higher temperatures and increased sunlight, system effectiveness is high year-round. Applications for colder regions for example, can however also be very effective, as has been shown in Wastewater Gardens projects in New Mexico and Poland in high elevation sites with long winters. However, in colder climates, sizing per resident must be larger to accomplish similar treatment.
Wastewater Gardens® are not only especially recommended for use in on-site systems, close to the facilities they are to service, but have also demonstrated their responsiveness in areas with groundwater close to the surface, for sites with rocky or impermeable clay soils (that often prevent standard leachfields from operating) and especially in sensitive areas close to rivers, lakes and coastal waters.
Graywater treatment
Graywater refers to wastewater other than that from the toilet (black water): this includes laundry machines, showers, sinks and kitchen water. These can be separated from blackwater, pass through a sedimentation tank instead of a septic tank before going to subsurface irrigation trenches if the local regulations permit it, and the separation is easy enough to be economically done. The advantages of separating graywater is that more irrigation can be accomplished with the wastewater and overall project costs will be lower, as the Wastewater Gardens® will treat a smaller quantity of water. In these cases, the Wastewater Gardens® treat just the blackwater, or both the blackwater and the kitchen's wastewater which contains food particles, grease and oil. However, we often work in situations where the separation between black and grey waters is too difficult and expensive, just as a "retrofit" to existing plumbing. It is then best to let the WWG treat both types of water, as Wastewater Gardens® can be designed to handle both.
Stormwater treatment
In urban settings and in regions subject to flooding during storms, constructed wetlands are being employed to clean the water and to slow down the movement of water which creates flood pulses. Stormwater from urban and paved surfaces often contain pollutants like oil and fuel residues which can be readily cleansed in a wetland. In addition, in areas where freshwater supplies are limited or expensive, making use of the stormwater enables the greening of the landscape using a natural and renewable resource.
Space requirements
On average we assume normal wastewater generation of 125-200 litres per person per day (although this number can vary greatly according to cultural norms and geographical location) with 2.5-4 m2 of WWG per person. These numbers however depend on many factors including the climate (the warmer the climate, the smaller an area is needed since the plants and microbes are more effective year-round), the number of people and amount of wastewater they produce, whether the system treats all the wastewater or whether the graywater is recycled separately, and what standard of treatment is required or desired. In cold climates, these numbers may be twice or three times as large, depending on the level of treatment required during the cold periods of the year when plants may be dormant and bacterial activity is slower.
Maintenance
Our staff provides on-site training of local staff and a maintenance manual upon completion of the project. Proper functioning of the systems is dependent on several simple but important maintenance steps:
- Septic Tanks: septic tanks need normal maintenance: filters need checking every 3-6 months and washing/rinsing if necessary. The septic tank should be pumped out when solids fill more than half its depth (a standard requirement for septic tank maintenance).
- Gravel: if porosity of the original gravel declines (which can occur decades later), new gravel can be substituted or the original gravel removed and cleaned. The plants can then be transplanted back in, and the system can continue effective treatment for decades more.
- WWG Water Level: Water levels in the wetland cells need to be checked periodically via the control box, especially during periods of low occupation when evapotranspiration may exceed input into the treatment wetland. Until plants become well established it is important that water levels not be allowed to drop below their root zone.
- Plants: the wetland plants need normal garden care - pruning for appearance and encouragement of new growth and flowers. Heavy prunings of plants should be removed from the WWG to prevent reduction of gravel porosity when that material decays. The prunings can be used for mulch outside the system or added to compost piles. Should the WWG be planted before connection to the septic tank (in absence of the sewage water's nutrients) it may be necessary to fertilize to help the plants get established and maintenance of adequate water levels with tap water is also required.
- Drainage: ensuring that drainage is adequate around the WWG so that runoff rainwater and soil do not wash into the system is extremely important. Wastewater Gardens® are built with a berm higher than surrounding ground level, but one must check occasionally to make sure soil has not built up around the WWG basin, which would allow rain runoff to enter.
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